This application is based up on and claims priority to Japanese Patent Application Number 09-216050 filed Aug. 11, 1997, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical device in general and more particularly to an acousto-optic device that may be used in an optical transmission system to add, drop and modulate selected wavelengths.
2. Description of the Related Art
In recent years, progress to a highly sophisticated information society has generated a tremendous amounts of information and an optical communication system using an optical fiber has been introduced as a way of transmitting such information. With this optical communication system, the transmission capacity has been increased year by year with the realization of a high speed modulation rate. A modulation rate of Gb/s or higher has already been put into practical use.
However, the need for a transmission systems which can transmit a large amount of data, such as that from image information, is expected to increase in the future. Such a high capacity system may be now required to have the transmission capacity of one Tb/s or more. The current systems cannot satisfy the requirement for the above transmission capacity only by improving the modulation rate. Therefore, an optical wavelength multiplex transmission/communication system is considered indispensable, and there have been attempts in recent years to introduce such a system.
An important element for realizing optical wavelength multiplex communication is the optical wavelength filter. This filter can combine onto a single optical fiber light beams of different (perhaps many different) wavelengths respectively generated by different light sources and can branch light beams of the different wavelengths transmitted through the single optical fiber to respective different fibers and detectors. The filter is thus a key device of the optical wavelength multiplex transmission system. The filter is required to satisfy different requirements depending on the system in which it is used. For example, the filter should be able to work with different numbers of wavelengths, from several wavelengths to about 100 wavelengths. The filter should be able to work with different wavelengths interval, from 1 nm or less to several tens of nm. The filter should be extremely low cost for application to an access system.
There are several devices which utilize mutual interference between an acoustic wave and an optical beam. FIG. 1 is a perspective view of such a device, in which Ti metal is thermally diffused into an X-Y cut LiNbO3 substrate 2 to form a channel waveguide 1, and a flat waveguide. On the substrate 2, this device has a waveguide lens 3 and a transducer 4 formed from a comb-tooth type electrode for exciting a surface acoustic wave (SAW).
In this device, a light beam is converted to a parallel light beam by the waveguide lens 3. A SAW generates a refractive index grating from the photo acoustic effect of the SAW. The light beam is diffracted by this grating into different directions depending on the frequency of SAW. When this diffracted light beam is condensed by lens 5, the diffracted light beam is focused to different points because the device functions as an optical deflector.
FIG. 2 is a top view of another example of a related art device utilizing the refractive index grating created by a SAW. This device, a collinear AO module with an inhomogeneous SAW waveguide, was present at the Photonics in Switching conference, at Sendai, Apr. 21-25, 1996. In this device, parallel optical waveguides 7, 8 are formed on a Y cut LiNbO3 substrate 6 and a thin film 9 formed of Ta2O5 is formed on the substrate as a SAW waveguide. In operation, even number mode light and odd number mode light are combined by the refractive index grating between the parallel optical waveguides 7, 8. As before, the SAW creates the refractive index grating. A selected wavelength of a light beam incident to the optical waveguide 7 is switched to the optical waveguide 8. The selected wavelength corresponds to the refractive index grating created by the SAW. In this device, the grating is weighted through a change in width a(z) and thickness h(z) of the thin film 9 which guides the SAW. The thin film 9 reduces a siderobe in the optical waveform. Moreover, a device in which weighting is realized by forming the SAW waveguide crossing the optical waveguide is also known.
FIG. 3 is a top view of an optical waveguide device which extracts a light beam having a selected wavelength and executes modulation by rotating the main axis of the waveguide refractive index for the selected wavelength to thereby rotate the polarization of the selected wavelength. The selected wavelength corresponds to the frequency of the SAW generated in the device. Optical waveguides 11, 12 are formed by diffusing Ti in a X cut LiNbO3 substrate 10 and creating deeply diffused regions 14 of Ti on both sides of a region 13 for guiding a SAW generated by a SAW transducer 15. To generate the SAW transducer (IDT) 15 is provided with a radio frequency (RF) signal.
The TE (transverse electric) and TM (transverse magnetic) mode beams of an incident light beam are isolated by a crossing type polarization beam splitter (crossing type PBS) 16, and thereby the TE mode beam is incident to the optical waveguide 12, while the TM mode beam is incident to the optical waveguide 11. In optical waveguide 11, the light beam of a selected wavelength corresponding to the SAW is converted from the TM mode to the TE mode through rotation of polarization. In optical waveguide 12, the TE mode beam of the selected wavelength is converted to a TM mode beam through rotation of the polarization.
In this example, the TM mode beams of non-selected, non-rotated wavelength light are output from the optical waveguide 11 to the non-selected beam side via a crossing type PBS 17, while the TE mode beam of the selected wavelength is output from waveguide 11 to the selected beam side. In optical waveguide 12, the TE mode beams of non-selected, non-rotated light are output to the non-selected beam side and the TM mode beam of the selected wavelength is output to the selected beam side. Thereby, the selected wavelength can be extracted and modulated using this optical waveguide device. In the FIG. 3 device, absorbing bodies 19 and 20 are SAW absorbing bodies for preventing the SAW from being reflected at end faces of the substrate.
In the FIG. 3 device, the SAW is propagated at a higher rate in the deeply diffused region 14 of Ti than in the substrate due to the influence of Ti. The SAW is thus trapped and propagated in the region 13 where the propagation rate of the SAW between deeply diffused regions 14 of Ti is rather low. Therefore regions 14 function as a SAW waveguide.
In the filter and switch of the optical waveguide shown in FIG. 1 or FIG. 2, where even and odd number modes are coupled, filtering and switching can be realized independently respectively for the TE mode beam and the TM mode beam. This can occur because the SAW is generated or formed with perfect symmetry, but the propagation constants of the TE mode beam and the TM mode beam in the optical waveguide formed on the plane are generally different.
Moreover, coupling between two adjacent optical waveguides is believed to depend on the polarization of the TE and TM mode beams. Therefore, filter and switch characteristics also depend on the polarization. This causes a problem in a device which is required to control a light beam having a desired polarization, which requirement may be present in an optical communication system.
To eliminate the polarization dependency, it has been proposed to use two pairs of the FIG. 1 and FIG. 2 devices to isolate the input beam by polarization. These two paris would correspond to the desired polarization. However, it is not practical to use two pairs of devices. On the other hand, in FIG. 3, the TE mode beam is converted to a TM mode beam and the TM mode beam to a TE mode beam by isolating polarization modes. Therefore, the dependence on polarization can be eliminated. However, to prepare the device, a time as long as several tens of hours is required to form the deeply diffused regions 14 of Ti Another problem with the FIG. 3 device is that regions 14 cannot be formed to cross the optical waveguides as shown in FIG. 2. Regions 14 cannot be formed on the optical waveguides because deeply diffused Ti erases the optical waveguides, which may also be formed by Ti diffusion. Without formation on the optical waveguides, the siderobes in the waveform cannot be reduced. Moreover, in a SAW waveguide formed from deep diffused regions 14 of Ti, the trapping force for the SAW is weak, and therefore, the SAW waveguide is ineffective.
These three devices demonstrate that a device which operates with TE/TM mode conversion (FIG. 3) is known and devices having a thin film (FIGS. 1 or 2) are known. Although these devices have been known for some time, no one has ever been able to form a thin film on a TE/TM mode conversion device, as mentioned above.
In order to successfully form a SAW waveguide, the sound velocity within the guide, where the acoustic waves are to propagate, must be less than that at peripheral portion thereof. For this purpose, the waveguide must intuitively be formed with a material assuring lower sound velocity. However, the speed of sound is as high as about 6000 m/s in material, such as SiO2 and Al2O3, which is often used for a buffer layer. It is intuitively not thought to be possible to form a higher speed film on such a buffer layer material. Moreover, sound velocity in LiNbO3 is different, to a large extent, depending on the propagation direction (about 3500 to 4000 m/s) and a material assuring the adequate sound velocity for the selected direction is required. Moreover, if a film is to be formed on the optical waveguide, the material used for thin film must be transparent and have a refractive index which is smaller than that of LiNbO3.
In addition, it is necessary to form, in the LiNbO3 substrate, a SAW having an amplitude sufficient for executing the TE/TM mode conversion. It is considerably difficult and requires a large amount of work to design a thin film having a material, thickness and width satisfying the various conditions. Moreover, in performing TE/TM mode conversion in a substrate, when films of different materials are formed on the surface of substrate, the surface conditions change and this is feared to create problems effecting TE-TM mode conversion. Therefore, films of different materials have not yet been attempted.
With the reason explained above, it has been thought that a device for guiding a SAW with a thin film to perform TE/TM mode conversion cannot be realized. It is apparent that a device having a waveguide formed in the z-axis direction, as in FIG. 1 and FIG. 2, cannot realize the TE/TM mode conversion performed by the device of FIG. 3.
Accordingly, it is an object of the present invention to provide an acousto-optic device which can add and drop light of a specific wavelength.
It is another object of the present invention to provide an acousto-optic device which is not dependent on polarization.
It is a further object of the present invention to provide an acousto-optic device which does not require a long period of diffusion to fabricate.
It is a still further object of the present invention to provide an acousto-optic device which propagates light having a reduced siderobe.
These and other objects are accomplished by providing an optical device having a substrate, a polarization beam splitter, first and second pairs of optical waveguides, a transducer and a thin film. The polarization beam splitter is formed on the substrate and has input and output sides. The first and second pairs of optical waveguides are formed on the substrate to guide polarized optical signals. The first pair of optical waveguides meets at the input side of the polarization beam splitter, and the second pair of optical waveguides meets at the output side of the polarization beam splitter. The transducer is formed of a comb-tooth electrode on the substrate to excite a surface acoustic wave on the substrate and rotate the polarization of the optical signal. The thin film covers a portion of each waveguide of either the first or second pairs of optical waveguides. The thin film may be formed of silicon dioxide or indium dioxide, either with a metal oxide optionally added thereto. The speed of sound in the thin film is less than that in the substrate.
This speed of sound means the speed around the film (mainly, the speed under the film). The thin film is formed of a material which is transparent to the optical signal. The thin film has a refractive index smaller than that of the optical waveguides.
The SAW waveguide, of the present invention which can effectively guide a SAW, can be simply formed by providing a transparent thin film of SiO2to a thickness of about 1 xcexcm or less on an LiNbO3 substrate. The thin film forms a ridge type structure in approximately the same direction as the optical waveguide formed within the substrate. This device propagates the light beam approximately in either the X or Y direction.
The SAW is thought to be trapped because of the additional mass on the substrate from the film, the change of surface conditions and the trapping effect of the ridge type structure at the surface of the LiNbO3. The SAW is not necessarily thought to be trapped by the different sound velocity in the thin film.
In the SAW guide formed as above, rotation of the crystal axis is sufficient for TE/TM mode conversion.
Moreover, by properly choosing the thickness, width and material of the film, the trapping strength of the SAW waveguide can be enhanced to a large extent compared with that of the related art device, and the strength and velocity of the SAW can be controlled to have the same distribution in both the width and traveling directions. The present invention provides wide flexibility in design and realizes sophisticated functions of TE/TM mode conversion in a SAW device.
With the optical waveguide device of the present invention, it is possible to realize a wavelength filter which is tunable to control the target wavelength. The present invention is indispensable and can be used as an add drop multiplexer (ADM), an optical cross connect and an optical exchange in an optical transmission system.
The present invention can focus on a single wavelength and act as a switch for that wavelength, thereby enabling modulation of that wavelength. Not only can the present invention simultaneously switch and modulate different selected wavelengths, but it can also function as an ASE light elimination filter to eliminate an ASE beam generated by an optical amplifier, thereby improving receiver sensitivity even after transmission over a long transmission line.
Because the filter of the present invention is tunable, can be mass produced, and can be produced for a low cost, the present invention is applicable to a wide range of optical transmission and processing systems. The present invention can enable combining and branching filters, provide switching and modulating functions, and improve wavelength characteristics.